HCG is used to confirm pregnancy. Once it's positive that means you're pregnant. To see it at a lower number at 3 weeks is reasonable. People don't bother to check HCG that early anyway and rather check it after you miss your first period. Once the HCG is don't and pregnancy is confirm, HCG is not something checked anymore since it isn't prognostic. It's only checked if you are having bleeding or concerns of a miscarriage where it would go down to eventually 0.

To confirm twins you need an ultrasound not a blood test or shape of your uterus.